WO03/094112 discloses a local color correction approach. An input image represented by a set of input pixel values is color corrected by locally modifying the input pixel values according to pixel neighborhoods to obtain an output image which has a set of output pixel values. Each set of output pixel values equals a non-linear combination of a set of input pixel values and its corresponding set of modified pixel values.
In one embodiment of this prior art, the input pixel values are low-pass filtered to obtain a tone mask which is an image which contains numerical values which are indicative of shadows, highlights, and midtones in the received digital image. For each one of the input component color values (usually red, green, and blue) of an input pixel, the corresponding output color values are obtained by applying an exponential function on the input color values. The masked values of the input component color values are used to vary the exponent of the corresponding exponential function. Thus, the output color values are obtained by applying a pixel-wise gamma correction, wherein each pixel has its own particular gamma value due to its particular masked value. These local gamma values are determined by the tonal properties of the pixel and its surrounding pixels. It is disclosed that other non-linear functions may be used to determine the output color values as a function of the input color values, such as sigmoidal or hyperbolical functions. In another embodiment, the sigmoidal functions are used for masked values representing middle gray, while the exponential functions are used for masked values, which represent black and white.
Although this approach increases the visibility of image content in the extreme ends of the dynamic range, it also increases the noise in these parts of the image content, and further tends to decrease the contrast of the image by increasing the brightness in dark parts of the image and by decreasing the brightness in bright parts of the image.